Selective oxidation of hydrocarbons is an important process in the chemical industry, as it helps in the production of many useful chemicals like alcohols, aldehydes and carboxylic acids. Though there were innumerable numbers of publications/patents that deal with this area, it still remains a very important research challenge as many processes need green chemical routes and cost effective manufacturing. As a result, there is a drive to develop green and efficient processes for the oxyfunctionalization of hydrocarbons. Usually; efficient activation of an alkane requires precious metal catalysts and strong oxidizing agents (HNO3, TBHP, and H2O2). Currently, partial oxidation processes are conducted either in gas phase or liquid phase using homogeneous/heterogeneous catalysts. Since molecular oxygen is cheap and abundant, it is the most practicable oxidant for partial oxidation processes. But, most of the heterogeneous catalyst based processes offer poor selectivity to the desired product when molecular oxygen is used as oxidant. Large quantities of energy are needed to separate the desired product from unwanted side products, leading to not only waste generation but also inefficient use of starting materials. Hence, achieving desired product selectivity remains an important task.
Adipic acid (AA) is an important selective oxidation product that is obtained from cyclohexane. Major part of AA produced is used as a precursor for the synthesis of Nylon-6,6. In addition, AA is widely used for the production of polyesters, polyurethane resins, plasticizers in the production of polyvinyl chloride (PVC) and polyvinyl butyral (PVB). Present day processes for AA involve multiple steps and also use highly corrosive acids. Thus, developing novel, clean and green routes for AA production is an important research theme.
The current industrial process for AA production is based on the catalytic oxidation of a mixture of cyclohexanol and cyclohexanone which is referred as KA oil (Ketone/alcohol). The KA oil in turn is obtained on partial oxidation of cyclohexane. Further oxidation of the KA oil to adipic acid is performed using 50-65% HNO3 as oxidant in the presence of Cu (II) and ammonium metavanadate as catalysts. The selectivity to adipic acid based on KA oil is very high yielding only small quantities of glutaric acid as by-product. But, main drawback of nitric acid oxidation process is the stoichiometric reduction of HNO3 to NOx in the form of greenhouse gas nitrous oxide (N2O). The amount of N2O produced is around 300 kg per tonne of adipic acid, which also depends on the amount of catalyst and composition of the KA oil used.
In addition to the commercial process, there are alternative routes for producing AA. For example, AA can be obtained by direct oxidation of cyclohexene using hydrogen peroxide using a phase transfer catalyst. It can also be prepared by dimerization of methyl acrylate, carbonylation of butadiene and by bio catalytic fermentation of glucose. The oxidation of KA oil also can be carried out with oxygen as the oxidant, in place of nitric acid, using catalytic amounts of Co and Mn acetate, at 70-80° C. in acetic acid as solvent. However, acetic acid as solvent poses severe corrosion problems, particularly when combined with the Mn and Co salts. Moreover, most of these approaches results in poor selectivity (30-50%) towards the desired product. In addition, use of soluble homogeneous catalysts leads to its leaching during the course of the reaction, hampering recycling of the catalysts.
Nitrogen doped carbons were reported to have diverse applications in catalysis, particularly as electro catalysts, photo catalysts and as heterogeneous catalysts. Nitrogen containing carbons have received greater attention for oxyfuctionalization of hydrocarbons, as they are able to activate oxygen molecules without the assistance of any metals.
Article titled “Metal-free activation of dioxygen by graphene/g-C3N4 nanocomposites: functional dyads for selective oxidation of saturated hydrocarbons” by X H Li et al. published in J. Am. Chem. Soc., 2011, 133 (21), pp 8074-8077 reports graphene sheet/polymeric carbon nitride nanocomposite (GSCN) functions as a metal-free catalyst to activate O2 for the selective oxidation of secondary C—H bonds of cyclohexane. By fine-tuning the weight ratio of graphene and carbon nitride components, GSCN offers good conversion and high selectivity to corresponding ketones. Besides its high stability, this catalyst also exhibits high chemoselectivity for secondary C—H bonds of various saturated alkanes and, therefore, should be useful in overcoming challenges confronted by metal-mediated catalysis.
Article titled “Boron- and fluorine-containing mesoporous carbon nitride polymers: metal-free catalysts for cyclohexane oxidation” by Y Wang et al. published in Angewandte Chemie International Edition, Volume 49, Issue 19, pages 3356-3359, Apr. 26, 2010 reports N-doped carbon materials can catalyze the oxidation of cyclohexane (CyH) with H2O2 as an oxidizing agent to produce the KA oil with >99% selectivity. The boron- and fluorine-enriched carbon nitride polymeric semiconductor synthesized by a facile one-step process using 1-butyl-3-methylimidazolium tetrafluoroborate as a soft template. The resulting materials show an advantageous “morel-like” mesopore structure (see picture) with narrow pore size distribution and good photoactivity under visible light. These materials are also good catalysts for the selective oxidation of cyclohexane.
Article titled “Nitrogen-, phosphorous- and boron-doped carbon nanotubes as catalysts for the aerobic oxidation of cyclohexane” by Y Cao et al. published in Carbon, Volume 57, June 2013, Pages 433-442 reports nitrogen-, phosphorous- and boron-doped carbon nanotubes (N-CNTs, P-CNTs and B-CNTs) prepared by a chemical vapor deposition method using xylene as carbon source and aniline-NH3, triphenyl phosphine and triethyl borate as nitrogen, phosphorous and boron precursors, respectively. N- and P-CNTs are active for the oxidation of cyclohexane in the liquid phase with molecular oxygen as oxidant. The highest mass-normalized activity, 761 mmolg-1 h-1, achieved over N-CNTs synthesized from aniline in an NH3 atmosphere, while the highest surface-area-normalized activity, 28 mmolm-2 h-1, was observed over P-CNTs. B-doping does not improve the activity of CNTs. The effect of the number of nitrogen functionalities and defects was investigated to reveal the structure—activity relationship of the doped CNTs.
Article titled “Graphite as a highly efficient and stable catalyst for the production of lactones” by Y F Li et al. published in Carbon, Volume 55, April 2013, Pages 269-275 reports that the carbon materials carbon nanotubes (CNTs), graphite, and activated carbon tested as metal-free catalysts. They showed excellent activity and selectivity in the Baeyer-Villiger (B-V) oxidation of cyclohexanone at room temperature using dioxygen (O2) as oxidant and benzaldehyde as sacrificial agent. Among them graphite found to be the most suitable for the formation of lactones from cyclic ketones, showing good recyclability and reusability. The use of the metal-free catalysts enables a green process for the production of lactones from ketones under mild reactions.
Article titled “Solvent-free and metal-free oxidation of toluene using O2 and g-C3N4 with Nanopores: Nanostructure Boosts the Catalytic Selectivity” by X H Li published in ACS Catal., 2012, 2 (10), pp 2082-2086 reports solvent-free oxidation of the primary C—H bonds in toluene to benzaldehyde has been achieved by using the metal-free catalyst g-C3N4 and O2. It is the nanostructure of g-C3N4 that boosts the high selectivity by tuning the homogeneous oxidation to hetergeneous oxidation and capturing all free .O2— radicals to effectively suppress the over oxidation of aldehydes.
Article titled “Selective catalysis of the aerobic oxidation of cyclohexane in the liquid phase by carbon nanotubes” by H Yu et al. published in Angewandte Chemie International Edition, Volume 50, Issue 17, pages 3978-3982, Apr. 18, 2011 reports Carbon nanotubes (CNTs) catalyze the aerobic oxidation of cyclohexane into cyclohexanol, cyclohexanone, and adipic acid with excellent activity and controllable selectivity. Nitrogen doped multi walled carbon nano tubes as catalysts gave higher yields of AA compared to gold catalysts. For instance, at 125° C. and 15 bar of O2 pressure, 45% cyclohexane conversion with 60% AA selectivity was observed.
However, high cost of the catalysts and requirement of solvent are the main drawbacks of these above catalysts. Therefore, there is need to develop a green process and a catalyst for the selective oxidation with improved yields.